Structural two-pack polyurethane adhesives composed of polyols and polyisocyanates have been state of the art for a long period. They cure rapidly after mixing and can therefore absorb and transmit forces after only a short time. Structural adhesives, also called construction adhesives or assembly adhesives, are used in the manufacturing industry to bond components to one another in such a way that the adhesive bond is part of a permanently durable construction. Adhesives of this kind are typically tough and elastic and have to meet high demands in relation to processibility, strength and adhesion forces.
The prior art discloses polyurethane adhesives formed from polyols and polyisocyanates, in which both the polyol pack and the polyisocyanate pack are liquid at room temperature, meaning that the glass transition temperatures and possibly melting points are below 23° C. For example, applications DE 102007062529 and EP 2655466 describe liquid polyurethane adhesives composed of at least one higher-functionality polyol pack. In this way, high final strengths are said to be achievable with simultaneously high elongation. These properties are necessary, for example, for the bonding of wind turbine half-shells.
Applications WO2009/015149, EP 2706075 and EP 2700666 likewise describe liquid two-pack polyurethane adhesives. As well as good metal and plastic adhesion, these structural adhesives are said to have a homogeneous modulus of elasticity over a broad temperature range and be suitable for bonds in the automotive sector.
The disadvantage of the above-described systems is comparatively low handling strength directly after the substrates have been joined. However, a high initial or handling strength is necessary to enable high cycle rates in production. For sufficient handling strength, a tensile shear strength of at least 0.3 MPa is required before the fixed components can be separated (A. Lutz, S. Schmatloch, “Strukturelles Verkleben von Faserverbundwerkstoffen im Fahrzeugbau” [Structural bonding of fiber composite materials in motor vehicle construction], lightweight design, March 2014).
In the case of the liquid two-pack polyurethane systems of the prior art, the cohesion of the adhesive bond arises only through chemical crosslinking of the polyol pack with the polyisocyanate pack and is therefore linked to the progress of the reaction. Therefore, the adhesive bonds cannot yet be subjected to mechanical stress directly after application and the substrates can move with respect to one another. Typical curing times before attainment of handling strength are 40 minutes to 6 hours (cf. A. Lutz, S. Schmatloch, “Strukturelles Verkleben von Faserverbundwerkstoffen im Fahrzeugbau”, lightweight design, March 2014).
The addition of catalysts can accelerate the curing; as well as the known tin-containing systems, for example, EP 2706075 describes a metal chelate complex which enables rapid and blister-free curing. However, the crosslinking and hence the handling strength are highly dependent on the choice of catalyst and the correct dosage.
In order to enable rapid further processing, the bonds with two-pack adhesives that are liquid at room temperature are additionally subjected to full-area or point precuring with the aid of thermal methods, for example by infrared heating or induction, at temperatures between about 60 and 120° C. The disadvantage of this process lies in an additional processing step.